Laboratory research: bed load guidance into sediment bypass tunnel inlet

Sediment management in reservoirs situated in mountainous regions is a critical operational concern with direct implications in live storage sustainability and therefore in production revenues. The paper presents the main results of a physical model study of a sediment evacuation system foreseen for a large hydropower scheme in Ecuador, carried out at the Laboratory of Hydraulic Constructions (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL). The system comprises a sediment bypass tunnel (SBT) and a submerged weir nearby the gated tunnel inlet, conceived mainly to bypass bed load sediments. Four operation modes were identified and documented. The final sediment bypass efficiency is almost 100% for frequent floods under upstream drawdown, reducing to 20% for larger flood events above the capacity limit of the bypass tunnel. The tests resulted in an improved configuration of the system in all operational modes, including reverse flushing. Guidelines for prototype operation were proposed to consider, among others, the sequential combination of high and low reservoir levels and different gate openings, during and after floods events, in order to improve the overall sediment bypass efficiency.

Influence of tributary widening on confluence morphodynamics

Marcelo Leite Ribeiro

Confluences are the nodes of the fluvial network. They are typically characterized by highly heterogeneous conditions that are favorable for a sound fluvial ecosystem, and play a key role in the connectivity of the river system. During the past centuries, many rivers and their tributaries have been channelized and regulated, often resulting in quasi-homogeneous flow and morphologic conditions, deficient flood safety, reduced natural dynamics, impoverished ecological value, and reduced connectivity. Marcelo Leite Ribeiro’s PhD research investigated the potential of river rehabilitation by means of a local widening in the tributary of channelized confluences in an alpine environment. Such confluences are characterized by a dominant discharge in the low-gradient main channel, a dominant supply of poorly graded sediment in the steep tributary, and a pronounced bed discordance at the tributary mouth. Due to considerably different confluence characteristics, none of the existing conceptual models can represent the hydro-morphosedimentary processes in this type of confluences. Dr. Leite Ribeiro’s research was performed in a laboratory set-up that is representative of the 20 major confluences on the Rhône River upstream of Lake Geneva, Switzerland. Measurements of the three-dimensional velocity field, turbulence, sediment transport, morphology and bed material characteristics in a wide range of configurations of the local tributary widening and under various hydraulic conditions provided the following results: - A conceptual model has been proposed for the hydro-morpho-sedimentary processes in the investigated type of channelized alpine confluences. This conceptual model explains the adaptation to changing hydraulic conditions and to the local widening of the tributary. - Without having adverse effects on flood safety, the local widening of the tributary was found to considerably enhance the hydraulic and morphologic heterogeneity, which enhances the ecological value. - The validity of this result for all investigated widening configurations indicates that relatively small interventions conditioned by local constraints can considerably contribute to the rehabilitation of confluences in strongly anthropologized environements.

EPFL - LCH2011

Hydro-morphodynamics of open-channel confluences with low discharge ratio and dominant tributary sediment supply

Sebastián Guillén Ludeña

River confluences in which the tributary supplies the dominant sediment load, and the flow discharge is abundantly provided by the main river, are typically observed in mountain-river basins. The existent knowledge on the hydrodynamic, morphodynamic and sedimentary processes involved in mountain-river confluences is sparse, since most of the studies on confluence dynamics focus on low land confluences. In this context, the present research study aims to deepen the knowledge on the hydro-morphodynamics and sedimentology of mountain-river confluences inspired in those of the Upper-Rhone river basin. For that purpose twelve laboratory experiments were conducted in two experimental facilities: one located at the Laboratory of Hydraulic Constructions of the École Polytechnique Fédérale de Lausanne (LCH-EPFL), and the other one located at the Instituto Superior Técnico de Lisboa (IST-UL). The experimental setup covered a wide range of configurations including three unit-discharge ratios, two junction angles, two sediment mixtures with different gradation coefficients, and two width ratios. The experiments were performed under movable bed conditions and with continuous sediment supply to both flumes, which represents a novelty in the study of this type of confluence. Systematic surveys of bed topography and water surface were recorded at different instants during the experiments and at equilibrium condition. Additionally and also at equilibrium, point velocities were measured and the spatial and grain size distribution of the bed sediments were analyzed. These measurements allowed the analysis of the influence of the discharge ratio, junction angle, sediment gradation and width ratio on the hydro-morphodynamics of open channel confluences. The bed morphology and hydrodynamics displayed some common features that were influenced by the variation of the tested parameters. The most significant morphological features were a marked discordance between the bed elevations of the tributary and main channel, a steep bed slope in the tributary, a bank-attached bar along the inner bank of the main channel and a scour hole that extended from the tributary mouth flanking the bar. The hydrodynamics were mainly characterized by a double flow deflection to the outer bank of the main channel by the tributary inflow and the bank-attached bar, flow acceleration downstream of the confluence, a shear layer that separated each confluent flows, and eventually supercritical flow regime in the tributary depending on the parameter setup. In summary, this research study presents a broad analysis of the hydrodynamic, morphodynamic and sedimentary processes involved in confluences characterized by low discharge ratios, and where the dominant sediment load is supplied by the tributary. This analysis contributes to widen the existent knowledge on the dynamics of this type of confluence and constitutes a benchmark for further studies, as well as a valuable tool for restoration projects.

EPFL2015

Improving hydropower flexibility by synergizing independently managed utilities

Sabine Chamoun, Steve Crettenand, Pedro Filipe De Almeida Manso, Giovanni De Cesare, Vincent Gaertner

Increasing the flexibility of hydropower allows operators to produce at optimal time ranges, not only dependent on the demand flow but also on the price trends. However, the possibility to improve hydropower flexibility can be hindered when assets are managed by different utilities. Therefore, finding synergies between operators of different assets is of great interest. The studied case consists of a highly interconnected system located in the Upper-Rhone Basin of the Swiss Alps. A wide range of possibilities was investigated to increase the system’s energetic capacity. The main concepts include the implementation of a new reservoir which can reach an altitude of 2230 masl, the heightening of existing reservoirs, adding pumping systems and creating new stages connecting the different structures. Rethinking existing hydropower systems is a must as in some cases improving the performance of a hydraulic system can be done by relatively small modifications but lead to greater efficiency.